I'm not sure what the parameters look like, but the deck needs to exhibit a combination of sturdiness and resilience underfoot to make for a comfortable running surface. Basically it should be springy but not too springy, and only in the normal direction. The frame needs to be solid and massive, again so that the whole thing doesn't move in unwanted directions while using it. The motor should be strong enough to operate smoothly at speeds up to at least 10 to 12 MPH, preferably more to allow for headroom.
Feature-wise, a good treadmill supports inclination of several degrees in the 'up' direction and at least a couple of degrees downward. Obviously the controls need to be responsive and easy to work with while running. And the whole thing needs to be designed without forehead-slapping engineering errors like the ability (much less the tendency) to pull objects beneath the deck. If the belt is exposed at the rear without a cover or guard of some kind, as was notoriously done by Peloton, that would be an example of how not to do it.
Basically, any treadmill that doesn't suck is going to end up weighing a few hundred pounds and costing several thousand dollars. It will be designed with gym use in mind, rather than primarily for home users.
I had a Precor C964 for several years, but sold it when I moved. I eventually replaced it with a similar model from the same company (TRM 425), and I'd say those two models are examples of very good commercial-grade treadmills that will last more or less forever in a home environment. Frankly I liked the older model a bit better, as it had simpler controls with less lag.